Hi eeuwe
In order to add the functionality that has this PO PRO screen, we had to stop using the post_types list by default.
Now, the Purchase Orders Overview is based in ATUM_List_Table , that at the same time is based in WP_List_Table.
So, in order to add new columns, you should use custom development, but as we don't offer this service you will need to hire a developer.
Best Regards,
Andrea.